A sunspot is formed as a result of the internal magnetic field
bursting through the visible surface and out into the corona.
Sunspots appear dark because the magnetic fields get in the way of
energy and heat being transported from inside the Sun to its
surface. Because these areas are heated less, they are a few
thousands of degrees Kelvin cooler (1,000-2,000K normally), making
them appear dark in comparison to the very bright rest of the
surface.
Sunspots are generally where flares and CMEs originate
from. They are therefore used as marker of solar activity. Lots of
sunspots generally means lots of solar flares and CMEs. Few or no
sunspots mean little or no solar activity.
Read more...